What makes grass grow?

Hi there, I did a search to try and find this answer, its either such common knowledge that it has no need of a topic or I just didnt dig deep enough, because im sure theres alot of people that know the answer.

Plain and Simple, what causes dirt blocks to grow grass on them?

I know they need light, from what Iv read about trees, torches will work for this (tho iv used sunlight thus far). But Iv built floating islands that never grow grass. Iv built dirt areas on the ground that have never grown grass. iv got several experiments going on now to see what it requires, almost all of them are failers.

The only thing so far that seems to make grass grow is if the dirt block is touching another dirt block that has grass on it, and I dont know if thats just coincidence or what.

I have several theorys and experiments going to test, But im getting no where so I just thought I would ask.

I know there is no set time limit because ive seen it grow instantly after placing it. So there's a part of me that wants to say its a randomized timer that sets off when placed, but i could be wrong, because i am frequently.

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Iv done some tests of my own to confirm what that "how grass grows guide" says. While growing grass is random and some times very very slow (several ingame days of dirt before grass appears with only 1 grass square near it, with me spending all day and night building a stuff near it so It wont become "chunks")

I dont think you can have grass grow 2 blocks high from dirt like the guide says. I have several pillars of 2 block high dirts setting around with 8 blocks of grass around the base (9 if the block there setting on counts) Basically this;

[] [] [] []
[] [] [] []

wont become this:

[] [] [] []
[] [] [] []

Maybe the way grass grows has changed sense the guide was made.

Also I dont know exactly what "level 4 lighting" is but the moon must give off at least that much.

I think it requires light, AND there must be an adjacent dirt block that has grass. Thats why grass wont grow on the top block in that diagram, because it is more than 1 block away from dirt that has grass.